1. Field of the Invention
The present invention relates to a communications device used in a mobile communications system using CDMA, and more particularly, to a base station device used in a mobile communications system using CDMA.
2. Description of the Related Art
In recent years, development of a digital cellular wireless communications system using CDMA (Code Division Multiple Access) technology, especially, DS-CDMA (Direct Spread-CDMA) technology has been pushed forward as a next-generation mobile communications system implementing a wireless multimedia communication.
In a CDMA communications system, codes are allocated to respective channels or users, and the respective channels or users are identified with the allocated codes in normal cases. Accordingly, a plurality of communications are simultaneously implemented by using the same frequency in a CDMA communications system.
However, if a plurality of communications are simultaneously made, signals of respective channels interfere with one another. As a result, the number of channels (a channel capacity) that can simultaneously make a communication is restricted. Here, interference among signals depends on the transmission powers of the signals. Accordingly, if the quality of communication environment is good, the interference among signals is reduced by decreasing the transmission powers, so that the channel capacity is increased. This technology is sometimes called TPC (Transmitting Power Control), and also has an effect of suppressing the power consumption of a mobile station.
Additionally, a technology for preparing a plurality of modulation methods, and for adaptively selecting a modulation method to be actually used between a base station and a mobile station depending on a communication environment has been studied in recent years. This adaptive modulation technology is described below.
With adaptive modulation in a CDMA communications system, a modulation method is adaptively changed based on a quality (such as a signal-to-interference ratio, an error rate, etc.) of communication environment between a base station and a mobile station in normal cases. As modulation methods, for example, QPSK shown in FIG. 1A, 16QAM shown in FIG. 1B, and 64QAM shown in FIG. 1C are prepared. Here, with QPSK, four signal points are defined on a phase plane, and 2-bit data is transmitted for each symbol. With 16QAM, 16 signal points are defined on a phase plane, and 4-bit data is transmitted for each symbol. With 64QAM, 64 signal points are defined on a phase plane, and 6-bit data is transmitted for each symbol. For example, data is transmitted with QPSK if a communication environment is not good, or data is transmitted with 16QAM or 64QAM if a communication environment is good. In this way, an efficient data transmission is implemented.
FIG. 2 is a block diagram showing an existing transmitting device that can make adaptive modulation. Here, assume that this transmitting device multiplexes user data and control data, and transmits the multiplexed data. Note that this transmitting device is a base station device provided in a mobile communications system.
A data flow rate monitoring unit 1 monitors the data rate of user data to be transmitted. A coding unit (CHCOD) 2 encodes the user data to be transmitted according to a predetermined coding method. A variable rate controlling unit 3 temporarily holds the user data, and outputs the user data at a corresponding rate while referencing a monitoring result of the data flow rate monitoring unit 1. An adaptive modulating unit 4 determines a modulation method based on channel quality information. The channel quality information represents the state of a channel between a base station and a mobile station. For example, a signal-to-interference ratio is used as the channel quality information.
A spreading unit 10 modulates the user data according to the modulation method determined by the adaptive modulating unit 4, and spreads the modulated data. Specifically, a mapping unit 11 arranges the user data at corresponding signal points according to the modulation method determined by the adaptive modulating unit 4. A spread code generator 12 generates a spread code allocated to the user data to be transmitted (or a mobile station receiving the user data). A spreader 13 multiplies the output of the mapping unit 11 by the spread code generated by the spread code generator 12.
A spreading unit 20 spreads control data. Configuration of the spreading unit 20 is fundamentally the same as the spreading unit 10. However, in the spreading unit 20, a spread code predetermined for control data is generated, and the control data is spread with the spread code. Basically, the control data is not adaptively modulated.
A code multiplexing unit 21 multiplexes the output of the spreading unit 10 and the output of the spreading unit 20, and outputs the multiplexed data. A receiving device (a mobile station in this case) receiving the signal transmitted from this transmitting device regenerates user data by using the same spread code as that used in the transmitting device.
An example of the adaptive modulation operations in the above described transmitting device is briefly described. Here, assume that user data is modulated with QPSK, and transmitted. Also assume that channel quality is good.
In this case, the transmitting device can selectively perform the following three operations.    (1) Reducing transmission power without changing a modulation method.    (2) Reducing a symbol rate to 1/2, and changing a modulation method from QPSK to 16QAM. In this case, since the number of bits per symbol is doubled, the data rate remains unchanged. In the meantime, since the symbol rate becomes 1/2, an increase in the transmission power can be consequently suppressed to some extent.    (3) Changing a modulation method from QPSK to 16QAM without changing the symbol rate. In this case, since the number of bits per symbol is doubled, the data rate becomes double. However, the transmission power becomes higher.
In a mobile communications system, an increase in a channel capacity, suppression of the transmission power of a communications device (especially, suppression of the power consumption of a mobile station), and a speedup in the data rate of user data are normally required. These requirements are satisfied by suitably selecting the above described (1) to (3).
With existing techniques, however, these requirements cannot be simultaneously satisfied. Namely, if attempts are made to hold the error rate of a data transmission to be a constant value, distances between signal points arranged on the phase plane shown in FIGS. 1A to 1C must be made constant. In the meantime, the transmission power is proportional to the square of a distance from the origin to a corresponding signal point on the phase plane coordinates, as is well known. Accordingly, if a signal point to be used increases, the transmission power naturally becomes higher. Assuming that distances between signal points are the same, the transmission power of 16QAM becomes higher than that of QPSK by approximately 6 dB, and the transmission power of 64QAM becomes higher than that of QPSK by approximately 12 dB.
As described above, if attempts are made to speed up a data rate with existing adaptive modulation techniques, this increases transmission power or power consumption. Or, if attempts are made to secure constant quality under predetermined transmission power, a data rate cannot be sped up in some cases.